The present invention relates to carbon nanotube field effect transistors (FETs), and more specifically, to improving electrical coupling between contact metals and carbon nanotubes in carbon nanotube field effect transistors.
Carbon nanotube field-effect transistors generally include a carbon nanotube that spans a gap between a source contact and a drain contact and serves as the channel of the transistor, the conductance of which is modulated by a gate separated from the nanotube channel by a dielectric material. There exists a contact resistance at the interfaces between the carbon nanotube and the source/drain contacts due, in part, to difficulties in coupling the cylindrical surface of the carbon nanotube to the contacts. High contact resistance at the interface between the carbon nanotube and either of the contacts will reduce current injection into the nanotube channel, thereby decreasing the performance of the transistor. This problem of contact resistance is exacerbated at technologically relevant nanotube diameters that are less than about 2 nanometers (nm) where a Schottky barrier presents itself at the contact metal/nanotube interface. The band gap of the carbon nanotube increases inversely with respect to diameter and a sufficiently large bandgap (˜0.6 eV) is necessary to attain a suitable on-/off-current ratio for digital applications. However, decreasing the diameter to achieve this bandgap leads to larger Schottky barriers and weaker coupling between the contact metal and the carbon nanotube thus increasing the contact resistance.